Bottom Line:
Although many chiral inorganic powders have been successfully synthesised, the artificial synthesis of chiral inorganic films is rare.The DNA-silica composite films exhibited strong optical activity at 295 nm and in the range of 400-800 nm, corresponding to DNA chiral packing (absorption) and to the helical blade in the impeller (scattering), respectively.The hierarchical structures were reflected from the surfaces by cross-polarised light, which confirmed that the films were strongly birefringent, with long-range anisotropy.

ABSTRACTThe circularly polarized reflection of nature is due to their distinct azimuthally twisted or helical character in the nanostructure of the surface films. Although many chiral inorganic powders have been successfully synthesised, the artificial synthesis of chiral inorganic films is rare. Herein, we reported a facile synthetic route for the growth of monolayered chiral film on the quaternary ammonium-modified silicon substrate. The films grew on the substrate surface because of the strong electrostatic interaction between positively charged quaternary ammonium groups and negatively charged phosphate groups of DNA, with subsequent growth to right-handed, vertically aligned, impeller-like helical architectures with left-handed two-dimensional square p4mm-structured DNA chiral packing. The DNA-silica composite films exhibited strong optical activity at 295 nm and in the range of 400-800 nm, corresponding to DNA chiral packing (absorption) and to the helical blade in the impeller (scattering), respectively. Upon removal of DNA templates, the pure inorganic impeller-like helical morphology was maintained; consequently, the scattering-based optical response was blue-shifted approximately 200 nm as a result of a decrease in the effective average refractive index. The hierarchical structures were reflected from the surfaces by cross-polarised light, which confirmed that the films were strongly birefringent, with long-range anisotropy.

f2: Macroscopic helical morphologies of CDSFs.Top view and side view of the low- and high-magnification SEM images of CDSFs formed on the silicon substrates without pretreatment (a1–4) and with H2SO4/H2O2 pretreatment (b1–4) showing the impeller-like helical architecture composed of several blades grown on the substrate. The molar composition of the synthesis gel was DNA (phosphate group):Mg2+:TMAPS:TEOS:H2O = 1:1.5:6:15:18 000.

Mentions:
Both the as-prepared CDSFs and the calcined CSFs were macroscopically smooth and glossy with the same appearance as a pure silicon wafer, which indicated that thin and uniform films can be formed on the silicon substrate (see Supplementary Figure S2). Figure 2 shows the scanning electron microscopy (SEM) images of the as-prepared CDSFs synthesised without and with the surface pretreatment. As shown in Figure 2a1–4, the film synthesised without the surface pretreatment was composed of particles with impeller-like helical morphology, which is evident in the incline-arranged blades from the top view and the half-impeller from the side view. The half-impeller had a uniform radius of ~2 μm and a uniform thickness of ~100 nm (i.e., the thickness of the blades corresponding to the length of DNA molecules). The blades of the vertically aligned impeller were stacked in a single/uniform direction, which revealed unambiguously the handedness of the macroscopic helical morphology. The impeller-like helical DNA–silica complex (IHDSC) with blades arranged in an anticlockwise manner in the side view (i.e., at an inclination angle smaller than 90° from right-bottom to left-top from the top view as shown insert in Figure 2a2) is defined as right-handed, and the opposite case (see Supplementary Figure S6) is defined as left-handed. We estimated the enantiopurity by counting the characteristic morphologies of more than 1000 randomly chosen particles in the SEM images, which were obtained from more than 20 different regions of the film surface according to the inclination angle of the blades. The CDSFs synthesised without the surface pretreatment were found to be predominantly right-handed with an absolute enantiomeric excess (ee) of approximately −90% (ee value was defined as 100% × [(l − r)/(l + r)], where l and r are the amounts of the left-handed and right-handed helical impellers in the given sample). The blades had similar inclination angles in the range of 36 to 46° (insert in Figure 2a2), and the corresponding pitch lengths of the helical blades were calculated to be 20–15 μm. The number of blades in each half-impeller ranged from 6 to 10.

f2: Macroscopic helical morphologies of CDSFs.Top view and side view of the low- and high-magnification SEM images of CDSFs formed on the silicon substrates without pretreatment (a1–4) and with H2SO4/H2O2 pretreatment (b1–4) showing the impeller-like helical architecture composed of several blades grown on the substrate. The molar composition of the synthesis gel was DNA (phosphate group):Mg2+:TMAPS:TEOS:H2O = 1:1.5:6:15:18 000.

Mentions:
Both the as-prepared CDSFs and the calcined CSFs were macroscopically smooth and glossy with the same appearance as a pure silicon wafer, which indicated that thin and uniform films can be formed on the silicon substrate (see Supplementary Figure S2). Figure 2 shows the scanning electron microscopy (SEM) images of the as-prepared CDSFs synthesised without and with the surface pretreatment. As shown in Figure 2a1–4, the film synthesised without the surface pretreatment was composed of particles with impeller-like helical morphology, which is evident in the incline-arranged blades from the top view and the half-impeller from the side view. The half-impeller had a uniform radius of ~2 μm and a uniform thickness of ~100 nm (i.e., the thickness of the blades corresponding to the length of DNA molecules). The blades of the vertically aligned impeller were stacked in a single/uniform direction, which revealed unambiguously the handedness of the macroscopic helical morphology. The impeller-like helical DNA–silica complex (IHDSC) with blades arranged in an anticlockwise manner in the side view (i.e., at an inclination angle smaller than 90° from right-bottom to left-top from the top view as shown insert in Figure 2a2) is defined as right-handed, and the opposite case (see Supplementary Figure S6) is defined as left-handed. We estimated the enantiopurity by counting the characteristic morphologies of more than 1000 randomly chosen particles in the SEM images, which were obtained from more than 20 different regions of the film surface according to the inclination angle of the blades. The CDSFs synthesised without the surface pretreatment were found to be predominantly right-handed with an absolute enantiomeric excess (ee) of approximately −90% (ee value was defined as 100% × [(l − r)/(l + r)], where l and r are the amounts of the left-handed and right-handed helical impellers in the given sample). The blades had similar inclination angles in the range of 36 to 46° (insert in Figure 2a2), and the corresponding pitch lengths of the helical blades were calculated to be 20–15 μm. The number of blades in each half-impeller ranged from 6 to 10.

Bottom Line:
Although many chiral inorganic powders have been successfully synthesised, the artificial synthesis of chiral inorganic films is rare.The DNA-silica composite films exhibited strong optical activity at 295 nm and in the range of 400-800 nm, corresponding to DNA chiral packing (absorption) and to the helical blade in the impeller (scattering), respectively.The hierarchical structures were reflected from the surfaces by cross-polarised light, which confirmed that the films were strongly birefringent, with long-range anisotropy.

ABSTRACTThe circularly polarized reflection of nature is due to their distinct azimuthally twisted or helical character in the nanostructure of the surface films. Although many chiral inorganic powders have been successfully synthesised, the artificial synthesis of chiral inorganic films is rare. Herein, we reported a facile synthetic route for the growth of monolayered chiral film on the quaternary ammonium-modified silicon substrate. The films grew on the substrate surface because of the strong electrostatic interaction between positively charged quaternary ammonium groups and negatively charged phosphate groups of DNA, with subsequent growth to right-handed, vertically aligned, impeller-like helical architectures with left-handed two-dimensional square p4mm-structured DNA chiral packing. The DNA-silica composite films exhibited strong optical activity at 295 nm and in the range of 400-800 nm, corresponding to DNA chiral packing (absorption) and to the helical blade in the impeller (scattering), respectively. Upon removal of DNA templates, the pure inorganic impeller-like helical morphology was maintained; consequently, the scattering-based optical response was blue-shifted approximately 200 nm as a result of a decrease in the effective average refractive index. The hierarchical structures were reflected from the surfaces by cross-polarised light, which confirmed that the films were strongly birefringent, with long-range anisotropy.